This invention relates to light emitting diodes (LEDs) and in particular LEDs having a broad uniform far field with high intensity which are highly adaptable for use as illuminating sources with electro-optic line modulators and electro-optic line printers.
In the prior art, semiconductors lasers and LEDs have been employed as a light source to produce an image on a photosensitive medium such as a xerographic photoreceptor used in a xerographic printer. In this kind of application, there is a need for uniformity of the intensity of light in the image as well as sufficient intensity of the light emitted. Further, if LEDs have to be utilized, it is necessary to provide a full width array of LEDs, one per picture element or pixel so that a line of light can be formed for discharge of the photoreceptor in an imagewise manner. Usually a plurality of light emitting device arrays are arranged in one or more rows and optical means is positioned between the photosensitive medium and the light source array to focus the light from the array sources to a single line on the surface of the photosensitive medium. The light sources are selectively turned on and off to effect line-by-line exposure of the moving photosensitive medium.
Semiconductor lasers have also been used in the past have been used as such light sources for rotating polygon scanned printers because of their high intensity. They have not, however, been totally best suited for application in electro-optic line printers due to a lack of uniformity of light intensity. In particular, coherent laser sources possess a far field pattern containing regions of high intensity and of low intensity in the single beam far field. Such a variation in intensity across the beam output is not desired because the line exposure on the photosensitive medium will not be uniform. For this and other reasons, LEDs are more favored as a light source because they may be designed to provide a light output of uniform intensity with very short coherence length.
In some cases, LEDs in the past have not provided sufficient output power and intensity to accomplish in an efficient manner the task of exposing a charged and moving photosensitive medium. For this reason, LEDs as light sources for xerographic photoreceptor applications have lacked the output intensity level for good photoreceptor discharge and as a result, semiconductor lasers have been still favored in many cases as the light source for printer applications.
Beside the problem of sufficiency of LED intensity, the maintenance of light output uniformity among a plurality of LEDs, as alluded to above, is a recognized problem in the art. To insure that the intensity of the broad light emission from the array is uniform across an LED array, elaborate control systems have been designed to provide for light intensity uniformity as exemplified, for example, in U.S. Pat. No. 4,455,562. This patent utilizes a binary weighted duty cycle control to obtain substantial uniformity in the light emitted from each LED in the array of LEDs.
The highest power LEDs have been top emitter types, but they lack power density necessary for most printer applications, i.e., they lack from sufficient light density per aperture size.
A most recent advancement in the printer arts has been the concept of a total internal reflection (TIR) line modulator which is a solid state multi-gate light valve that may be used to address a photosensitive medium. The line TIR modulator comprises a crystal bar of electro-optic material with an array of interdigital electrodes deposited on one of its major surfaces, which electrodes, when electrically addressed, introduce or induce a periodic electric field into the bulk crystal. Each of the electrodes may be individually addressed by an electronic signal forming a signal pattern across the array. A broad or wide sheetlike beam of high intensity light is required for the line modulator. The beam is introduced into the crystal at an angle incident to the plane of the major surface incorporating the electrodes. An example of the line TIR modulator is disclosed in U.S. Pat. No. 4,281,904 to Robert A Sprague et al.
To carry out the exposure process of the photosensitive medium, a sheetlike beam of light is transmitted through the electro-optic element of the TIR line modulation at a slight angle relative to the optical axis of the light to cause total internal reflection at the internal surface incorporating the electrode array. Successive sets of digital bits or analog samples, representing respective collections of picture elements or pixels for successive lines of an image, are sequentially applied to the electrode array. Localized electric bulk or fringe fields developed in the crystal in proximity to the TIR incidence of light modulate the light and change the phase front of the sheetlike light beam in imagewise configuration onto the charged photosensitive medium. Examples and teachings relative to electro-optic line printer applications may be found in U.S. Pat. Nos. 4,367,925; 4,369,457; 4,370,029; 4,437,106; 4,450,459; 4,480,899 and 4,483,596.
What is needed for the electro-optic line modulator and its printer application is a single solid state light source capable of providing a sheetlike, uniform and high intensity beam of light.